Date of this Version
PHYSICAL REVIEW X 4, (2014), pp. 011030. DOI: 10.1103/PhysRevX.4.011030
Although CO2 and SiO2 both belong to group-IV oxides, they exhibit remarkably different bonding characteristics and phase behavior at ambient conditions. At room temperature, CO2 is a gas, whereas SiO2 is a covalent solid with rich polymorphs. A recent successful synthesis of the silicon-carbonate solid from the reaction between CO2 and SiO2 under high pressure [M. Santoro et al., Proc. Natl. Acad. Sci. U.S.A. 108, 7689 (2011)] has resolved a long-standing puzzle regarding whether a SixC1−xO2 compound between CO2 and SiO2 exists in nature. Nevertheless, the detailed atomic structure of the SixC1−xO2 crystal is still unknown. Here, we report an extensive search for the high-pressure crystalline structures of the SixC1−xO2 compound with various stoichiometric ratios (SiO2∶CO2) using an evolutionary algorithm. Based on the low-enthalpy structures obtained for each given stoichiometric ratio, several generic structural features and bonding characteristics of Si and C in the high-pressure phases are identified. The computed formation enthalpies show that the SiC2O6 compound with a multislab three-dimensional (3D) structure is energetically the most favorable at 20 GPa. Hence, a stable crystalline structure of the elusive SixC1−xO2 compound under high pressure is predicted and awaiting future experimental confirmation. The SiC2O6 crystal is an insulator with elastic constants comparable to typical hard solids, and it possesses nearly isotropic tensile strength as well as extremely low shear strength in the 2D plane, suggesting that the multislab 3D crystal is a promising solid lubricant. These valuable mechanical and electronic properties endow the SiC2O6 crystal for potential applications in tribology and nanoelectronic devices, or as a stable solid-state form for CO2 sequestration.